1. Field of the Invention
The present invention relates to a pulsed Doppler radar system for detecting a target moving at a high speed.
2. Description of the Prior Art
Pulsed Doppler radar systems are widely used for searching and tracking targets. FIGS. 1 and 2 schematically show the construction of known chirp-type and code modulation-type pulsed Doppler radar systems, respectively. Each of the pulsed Doppler radar systems shown in these figures generally comprises a transmitter 1, a receiver 2, an antenna 3, a T/R switch 4, a pulse compressing unit 5, a pulse doppler processing unit 6, an amplitude detector 7 and a display 8.
In FIG. 1, a transmission pulse having a pulse width .tau. is generated by a pulse generator 11 in the transmitter 1 and expanded by a pulse expander 12 to a chirp (linearly frequency-modulated) pulse having a pulse width T (T&gt;.tau.) and a frequency band .DELTA.f (=1/.tau.) shown in FIG. 4a. In the meantime, in the radar system shown in FIG. 2, the pulse generator 11 in the transmitter 1 generates a transmission pulse having a pulse width .tau.. In the case of phase-modulation by a seven-bit Barker code, the generated pulse is converted to a phase-coded pulse having a pulse width T (FIG. 5a) by a phase-modulator 14 comprising delay elements 15 and an adder 16 shown in FIG. 5b.
The modulated transmission pulses produced in the chirp-type radar system as shown in FIG. 1 and the pulse modulation-type radar system as shown in FIG. 2 have extended frequency bands and pass through the T/R switches 4 and the antennas 3 to be emitted to targets. The transmitted waves are reflected by the targets and received by the antennas 3. The received signals pass through the T/R switches 4 and are supplied to the receivers 2 where the received signals are converted to complex digital video signals.
FIG. 3 shows an example of the construction of the receiver 2. A signal received by the antenna 3 is fed through the T/R switch 4 to a mixer 21 where the received signal is multiplied by the output of a stable local oscillator (STALO) 22 and converted to an IF (intermediate frequency) signal. The IF signal is amplified by an IF amplifier 23 and divided to two. One of the divided IF signals is supplied to a first phase-sensitive detector 241 and multiplied by the output of a coherent oscillator 25 to be phase-detected. The output of the coherent oscillator 25 is also supplied through a 90-degree phase shifter 26 to be delayed by 90.degree.. The 90-degree delayed signal is fed to a second phase-sensitive detector 242 and multiplied by the other of the divided IF signals to be phase-detected. The outputs of the respective phase-sensitive detectors 241 and 242 indicate real and imaginary parts, respectively, of a received complex video signal and stored in corresponding sample holders 271 and 272 and converted by analog-to-digital converters 281 and 282 to form a digital complex video signal to be fed to the pulse compressing unit 5.
Next, operations of the pulse compressing unit 5 and the pulse Doppler processing unit 6 in each of the radar systems will be explained with reference to FIGS. 1 through 8. A reference memory 50 (FIGS. 1 and 2) in the pulse compressing unit 5 has already stored a reference signal. Assuming that the modulated transmission pulse generated by the transmitter 1 (shown in FIG. 4a in the case of the chirp-type system or in FIG. 5b in the case of the code modulation-type system) is represented as TR(t), the reference signal RF(t) shown in FIG. 4b in the case of the chirp-type system or in FIG. 5c in the case of the code modulation system is expressed in the following: EQU RF(t)=TR(T-t).times.exp(-2.pi.f.sub.0 t) (0.ltoreq.t.ltoreq.T) EQU RF(t)=0 (T&lt;t)
where T denotes the width of a transmission pulse and f.sub.0 denotes a carrier wave frequency. If an amplification charcteristic for a received signal is indicated by w(t) shown in FIG. 4b, the reference signal is expressed as follows: EQU RFw(t)=RF(t).times.w(t)
The reference signal RFw(t) is a function of time t, and can be considered to be equivalent to a function R(r) of a range bin r (=t/ts) where ts indicates a sampling interval. When this function R(r) is Fourier-transformed in the direction of range, a spectrum of the reference signal Rr(fr) expressed in the following is obtained: EQU Rr(fr)=Fr[R(r)]
where Fr indicates a Fourier-transform in the direction of range. The spectrum Rr(fr) of the reference signal is prestored in the reference memory 50. It should be noted that the spectrum Rr(fr) is produced in the same manner in both chirp-type and code modulation-type systems.
Referring to FIG. 6, there is shown a flowchart of signal processing performed in the pulse Doppler processing unit 6. In a step S1, output data S(r) supplied from the receiver 2 and temporarily stored in a buffer memory 54 are Fourier-transformed in the direction of range by an N-point FFT operator 52, the output of which is expressed in the following: EQU Sr(fr)=Fr[S(r)]
where Fr indicates a Fourier-transform in the direction of range and fr is a frequency.
In a step S2, a complex multiplier 51 multiplies the spectrum Rr(fr) of the reference signal stored in the reference memory 50 by the output data Sr(fr) of the N-point FFT operator 52, and the result Ur(fr) expressed in the following is temporarily stored in a buffer memory 55: EQU Ur(fr)=Sr(fr).times.Rr(fr)
In a step S3, an N-point IFFT operator 53 performs an inverse Fourier-transform on the data Ur(fr) output from the complex multiplier 51 and stored in the buffer memory 55. The output of the N-point IFFT operator U(r) is expressed as follows: EQU U(r)=Ffr.sup.-1 [Ur(fr)]
where Ffr.sup.-1 indicates an inverse Fourier-transform in the direction of frequency.
In a step S4, the output data U(r) of the N-point IFFT operator 53 are stored in two-dimensional (2D) memory 61 with respect to a pulse hit p as data U(r,p), and an M-point FFT operator 62 performs an M-point Fourier-transform on the data U(r,p) in the direction of pulse hit before pulse Doppler processing is performed on the transformed data. The output of the M-point FFT operator 62 Up(r,fd) is expressed in the following: EQU Up(r,fd)=Fp[U(r,p)]
where Fp indicates a Fourier-transform in the direction of pulse hit and fd indicates a Doppler frequency.
The output of the M-point FFT operator 62 is envelope-detected by the amplitude detector 7 and a target is displayed on the display 8 by using a range-Doppler indication.
Next, a consideration will be made on the case where such a conventional pulsed Doppler radar system tracks a moving target which moves in a range-bin at a relative speed V resulting in a Doppler frequency fd. In the case of the chirp-type system, a pulse reflected by the moving target and received by the receiver has a time dependent frequency characteristic shown by a dotted line in FIG. 7. On the other hand, a pulse reflected by a target having a relative speed equal to zero has a time dependent frequency characteristics shown by a solid line in FIG. 7. Therefore, a frequency shift equal to the Doppler frequency fd between these characteristics can be seen. Since a delay-frequency characteristics of the pulse compressing unit of the conventional pulsed Doppler radar system is determined on the basis of the fact that a target moves at a relative speed equal to zero, a pulse reflected by that target has a shortened compressible range, and, as shown in FIG. 8, frequencies in a range only between an upper limit f.sub.1 and a lower limit f.sub.2 can be pulse-compressed. This leads to a loss equal to the output of the pulse compressing unit multiplied by (.DELTA.td/T). Further, the pulse compressing unit produces a delay .DELTA.td=(T/.DELTA.f)fd which corresponds to a decrease in display range by a range bin equal to .DELTA.td/.tau..
In the case of the code modulation-type radar system, a frequency shift fd in a Doppler frequency rotates the phase of a received pulse, which produces a shift in phase characteristics of the pulse before being compressed and the reference signal. This results in an increase in level of a range side lobe and such drops in pulse compression performances as a reduction in pulse compression rate and a positional deviation or separation of compressed pulses. As a result, a range detection of a target moving at a high speed may erroneously be performed or become impossible. For example, in the case where a Barker code is used for a code sequence, if a non-compressed pulse having a pulse width T is subjected to the phase rotation of 2.pi. due to the Doppler shift, an inconsistency in phase between the non-compressed pulse and the reference signal occurs. FIG. 8 shows an example of a pulse compressed waveform in the case of using a 13-bit Barker code. It can be said that the phase rotation of 2.pi. taken place in a non-compressed pulse having a pulse width T is equal to 2.pi., that is, 2.pi.fd.multidot.T=2.pi.[rad]. By substituting fd=2V/.lambda. (.lambda.: a wavelenght of a transmitted signal) for fd in the above equation, V=.lambda./2T can be obtained, which means that the pulse compression becomes impossible when V=.lambda./2T.